Well according to the forum stats there are 12537598 posts on this forum. Along with the posts text it has to store user information and other stuff, so I would estimate an average of about 1.5KB of storage per post. That works out to about 18.8GB, which I would think is the size of the database. Another 3GB for OS and another 1GB or so for SMF/other applications brings it up to around 22.8GB - thats my guess, not including logfiles/backups/caches.